Matti Orpana
Tensotech Consulting
Chydenius Center
FIN-67100 Kokkola
Finland

Abstract:
The most prominent material for stressed membrane structures is obviously the fabric. It is prominently present, attracts much attention and looks very simple. To obtain this pleasant charisma considerable research has been carried out. The material is analysed and specific properties defined and adapted. Properties like transparency, durability, fire retardance but also elasticity and strength. In this paper the fabric is discussed to get a better understanding of those properties. First the different weaves are discussed, followed by the different coatings The most common combinations of base cloth and coating are explained. Finally the structural behaviour of the fabric is discussed.

Threads
A thread is build up out of fibres. There are natural fibres and synthetic fibres. Natural fibres have a restricted length and are bound up in strands. These are the so-called spun fibres. Synthetic fibres theoretically have an endless length and are called filaments. The cross-section of natural fibres is smaller than 0.1mm, where synthetic fibres can have larger cross sections. The shape of the cross section is round for natural fibres but can have any shape in chemical fibres. For membrane structures it is best to have a yarn with a circular cross-section.
The mechanical properties of materials in the building industry are normally specified in N/mm2. In technical textiles this is not common because it is not easy to determine the cross section of a very small fibre. Therefore it is usual to determine the weight of a fibre with a certain length. When the specific mass is known from the fibre, it is possible to determine an average cross-section of the material. This mass-per-length unit is indicated with Titer with the symbol Tex: 1 Tex weight in grams per 1000m length. In synthetic fibres it is common to use decitex: 1 dTex= weight in grams per 10000m length.

A Polyester fibre for example with a Titer of 8.35 dTex has a weight of 8.35 grams at a length of 10000m. When the product is that small, it is very difficult to use it in industrial processes. Therefore it is spun into threads. One thread may be composed of hundreds of fibres. When a thread only has one fibre, it is called monofil. Spun fibres need to be stabilised by twisting around the centre of the thread. Filaments don't need it, but it facilitates the handling. The twisting influences the stress-strain behaviour of the threads. The more the thread is twisted the more the elasticity decreases compared to the elasticity of the fibre. By regulating the twisting, the mechanical properties of the thread can be determined precisely.
Filament threads are characterised according to the System Tex, where the number of fibres and twists are added. For example, a thread which is called 2200 dTex f 200 z 60 has a total Titer of 2200 dTex, made out of 200 fibres, the thread is twisted 60 times per meter in z direction.

There are several fibres that can be applied in membrane structures. For each project it is necessary to consider which type of fabric can be used. Several fibres do have the potential to be applied, however the high costs of it prevent a wide utilisation.

Cotton fibre
This type of fibre is the only organic fibre which is being used in membrane structures. Frei Otto used it for his early garden show structures and nowadays it is still applied in some rental tents. Because of the organic properties of the material it is subject to fungi and moisture. When used permanently it has an expected lifetime of about 4 years.

Polyamide 6.6 (Nylon)
The nylon fibre has a bad resistance against UV light, swells in the length direction when it gets wet and is therefore of little importance for textile architecture. It is frequently applied in the sailing industry because of the low weight and high strength.

Polyester
Polyester fibre together with fiberglass is the most common fibre in textile architecture and regarded as a standard product. The fibre has a good breaking strength and elasticity. Because of the considerable elongation before yield, the material is 'forgiving'. It enables small corrections to be made during installation. The mechanical properties of the material decrease with sunlight and there is ageing.

Fiberglass
The material fiberglass is made from is of course glass, from which threads are spun which have a certain bending capacity. The fiberglass has a high tear strength, but remains brittle and has little elastic stretch. Because of the brittleness the material needs to be handled carefully and needs very accurate manufacturing. The material is not subject to ageing which has a tremendous impact on the expected lifetime of the structure. But the tensile strength of the material decreases when it is subjected to moisture.

Aramid fibre
This is a relatively new type of fibre, discovered simultaneously by Akzo (Twaron fibre) and Dupont (Kevlar fibre). The material has a high tensile strength and is chemically resistant. A drawback is the low elastic stretch and the bad resistance against high temperature and UV-light.

Structure of the weave

Fabric that is used normally for membrane structures is built up from a weave, with a covering on both sides to protect it from water and pollutants. There are several ways to establish a connected weave. The basic method of weaving is called basket weave, where the weft threads pass the warp threads alternating above and under. There are many varieties possible, like passing three warp threads underneath and one above. All kinds of patterns occur as is done for nice carpets. But for structural use this it is not sufficient and therefore only the basket weave and Panama weave is used for membrane structures. Panama weave indicates that the weave operation is done with more than one thread at a time. 12*12 Panama means that one cm of fabric contains 12 warp and 12 weft threads. Panama bond has a better mechanical behaviour than basket weave because of the multiple yarns that are used.

Coatings

In the table above the fibres are described from which the fabric is woven. To create durable and watertight cloth most of the fibres need a coating on both sides. There are several coatings available. The most common ones are PVC coatings, Teflon coatings and silicone coatings. Sometimes a coating is not applied, but a foil is laminated upon the fabric.
The coating is often used to weld the different parts of the membrane together. The adhesion of the coating to the fabric is an indication of the strength of the seam. The adhesion of a lamination to the fabric is much lower and therefore requires other connection methods for the seams.

PVC coating on Polyester weave
This type of coating is mostly used on Polyester fabric. It is either coated or laminated upon the weave. Many different manufacturers provide such a material, which range from laminated fabrics for party rental tents to heavy coated fabrics for permanent (15-year replacement cycle) architectural installations. The fabric comes in numerous colours, has three different top coatings (Tedlar, Fluotop T, Acrylic) and is considered a fire-resitive material.

PVC coating on Aramid weave
Another interesting lightweight building material is Aramid fibre Air Tubes. These high pressure air tubes can take on the support function of a beam, an arch or a grid becoming a type of frame structure. The Aramid fibres are braided into curved forms and bonded to an inner urethane membrane to create seamless inflatable arches of approximate 30 psi. The Aramid fabric is enclosed with a PVC cover to protect the fibres from UV-degradation.

PTFE coating on fiberglass weave
Teflon coated fiberglass fabric is the most permanent of the coated architectural fabrics. First employed as a roof in 1974 for the La Verne College Student Centre in California it has a lifetime of over 30 years. It can be used only for permanent applications and is not relocatable. The fabric is considered non-combustible and as such meets the most stringent building codes world wide. When new it has an oatmeal appearance which bleaches to white after a couple of months in the sun. With translucency's up to 25% it has been used in such projects as the Georgia dome, Denver Airport and the Millennium dom.

Acknowledgements

Matti Orpana would like to thank Rogier Houtman for his collaboration on the preparation of this paper.
Rogier Houtman
Tentech Design&Engineering
P.O. box 619
NL-2600 AP Delft
The Netherlands